Im not an expert at this topic, but im pretty sure its because roblox uses scripts for the player’s control. If you were to run the game and look inside your playerscripts or character, you would find movement scripts that are automatically inserted by Roblox upon playing the game.
There are articles on this. You could probably reference the solution in this post to help guide you:
Unfortunately though, the short answer is roblox controls the players movement via a script inserted into their character, so you cannot just override it using userinputservice or bindtorenderstep.
EDIT: I also found this post on the forum, and this is a way you could change the controls too:
